1. Field of the Invention
The present invention relates to electronic watermark detection apparatus and methods. More specifically, the invention relates to apparatus and methods for detecting electronic watermarks in digital documents, even if the content of the digital documents has been subjected to distortion or other attempts to hide or destroy the watermarks.
2. Discussion of the Background
In recent years, electronic digital content, such as sound, music, active movies, and still pictures, have come to circulate widely. Much digital content is protected under copyright. In order to protect the copyright of the digital content, various electronic watermark methods are used.
An electronic watermark method comprises embedding an electronic watermark within the digital content so that detecting and extracting the digital watermark is difficult. According to the intended use, the electronic watermark indicates identification information of a copyright person or a user, rights information of a copyright owner, use conditions, secret information required at the time of use, copy control information, and so on.
The digital content in which the electronic watermark is embedded is often geometrically distorted by various normal operations of a user, or by intentional attack. As an example of user's normal operations, there is a change of the display size of an image. As an example of intentional attack of the user, there is a distortion of an image.
With regard to the electronic watermark method, it is required that the electronic watermark does not disappear, is not altered, and is able to be extracted, even if geometrical distortion is provided to the digital content. The requirement is called robustness.
Generally, geometrical distortion is classified into global distortion and local distortion.
Global distortion is distortion of a whole image, and may be caused by scaling, rotating, and/or parallel displacing. Global distortion can be expressed as an affine transformation formula. FIG. 1 illustrates how an original image I becomes a distorted image I′ by global distortion. A point a(x, y) of the original image I is displaced to a point a′(x′, y′) of the distorted image I′ by global distortion. In this case, scaling and rotating can be expressed with four parameters, each being a component a11, a12, a21, and a22 of a 2×2 matrix. Parallel displacing can be expressed with two parameters (b1, b2). Therefore, global distortion can be expressed with the following formula with these six parameters a11, a12, a21, a22, b1, and b2.                               (                                                                      x                  ′                                                                                                      y                  ′                                                              )                =                                            (                                                                                          a                      11                                                                                                  a                      12                                                                                                                                  a                      21                                                                                                  a                      22                                                                                  )                        ⁢                          (                                                                    x                                                                                        y                                                              )                                +                      (                                                                                b                    1                                                                                                                    b                    2                                                                        )                                              (        1        )            
On the other hand, local distortion means distortion in which each pixel of the whole original image is displaced by a different 2-dimensional vector. FIG. 2 shows that an original image I becomes a distorted image I′ by local distortion. Therefore, local distortion can be expressed with the following formula as 2-dimensional generalized-coordinate conversion.                               (                                                                      x                  ′                                                                                                      y                  ′                                                              )                =                  (                                                                      f                  ⁡                                      (                                          x                      ,                      y                                        )                                                                                                                        g                  ⁡                                      (                                          x                      ,                      y                                        )                                                                                )                                    (        2        )            where functions f and g are arbitrary functions. Although geometrical distortion may also include clipping, for brevity its description is not specifically included here.
The position of each pixel of the distorted image provided by geometrical distortion displaces the position of each pixel of an original image. An example of a detection method of electronic watermarks with consideration to displacing position of a pixel is shown in U.S. Pat. No. 6,108,434 (Cox et al.).
The Cox et al. method finds a best corresponding shifting position by repeatedly comparing with a predetermined block of an original image, while shifting a position of a block of a distorted image little by little on the basis of the position of the predetermined block of the original image. If the distorted image is shifted based on the best corresponding shifting position, the distorted image is concluded to correspond to the original image. Thereby, the shifting position of the distorted image corresponding to a original image can be compensated, and the electronic watermark in the distorted image can then be detected.
The Cox et al. method involves a large computational load for arithmetic calculations, since it is necessary to search for a large number of possible shifting positions. With regard to global distortion, as just to find the shifting position of a single arbitrary block, the Cox et al. method is considered to be an effective resolution method.
The geometrical distortion that a user performs on an image is local distortion in many cases. In practice, it is difficult to detect the shifting position of each pixel by local distortion.
In local distortion, since vectors of neighboring pixels are almost the same, neighboring pixels can be treated as a small block unit. Even if local distortion is treated as a small block unit, it is difficult in practice since the number of candidate block positions is a large and processing time becomes huge when using the Cox et al. method.
As explained above, a method for detecting electronic watermarks on local distorted images is not believed to be provided by known systems.